Carbon fibrils are vermicular carbon deposits having diameters less than 500 nanometers. They exist in a variety of forms, and have been prepared through the catalytic decomposition of various carbon-containing gases at metal surfaces.
Tennent, U.S. Pat. No. 4,663,230, describes carbon fibrils that are free of a continuous thermal carbon overcoat and have multiple graphitic outer layers that are substantially parallel to the fibril axis. They generally have diameters no greater than 0.1 micron and length to diameter ratios of at least 5. Desirably they are substantially free of a continuous thermal carbon overcoat, i.e., pyrolytically deposited carbon resulting from thermal cracking of the gas feed used to prepare them.
Tubular fibrils having graphitic layers that are substantially parallel to the microfiber axis and diameters between 3.5 and 75 nanometers, are also described in Tenant et al., U.S. Ser. No. 871,676 filed Jun. 6, 1986 (xe2x80x9cNovel Carbon Fibrils, Method for Producing Same and Compositions Containing Samexe2x80x9d), Tenant et al., U.S. Ser. No. 871,675 filed Jun. 6, 1986 (xe2x80x9cNovel Carbon Fibrils, Method for Producing Same and Encapsulated Catalystxe2x80x9d), Snyder et al., U.S. Ser. No. 149,573 filed Jan. 28, 1988 (xe2x80x9cCarbon Fibrilsxe2x80x9d), Mandeville et al., U.S. Ser. No. 285,817 filed Dec. 16, 1988 (xe2x80x9cFibrilsxe2x80x9d), and McCarthy et al., U.S. Ser. No. 351,967 filed May 15, 1989 (xe2x80x9cSurface Treatment of Carbon Microfibersxe2x80x9d), all of which are assigned to the same assignee as the present application and are hereby incorporated by reference.
Fibrils are useful in a variety of applications. For example, they can be used as reinforcements in fiber-reinforced composite structures or hybrid composite structures (i.e. composites containing reinforcements such as continuous fibers in addition to fibrils). The composites may further contain fillers such as a carbon black and silica, alone or in combination with each other. Examples of reinforceable matrix materials include inorganic and organic polymers, ceramics (e.g., lead or copper). When the matrix is an organic polymer, it may be a thermoset resin such as epoxy, bismaleimide, polyamide, or polyester resin; a thermoplastic resin; or a reaction injection molded resin. The fibrils can also be used to reinforce continuous fibers. Examples of continuous fibers that can be reinforced or included in hybrid composites are aramid, carbon, and glass fibers, alone, or in combination with each other. The continuous fibers can be woven, knit, crimped, or straight.
The composites can exist in many forms, including foams and films, and find application, e.g., as radiation absorbing materials (e.g., radar or visible radiation), adhesives, or as friction materials for clutches or brakes. Particularly preferred are fibril-reinforced composites in which the matrix is an elastomer, e.g., styrene-butadiene rubber, cis-1,4-polybutadiene, or natural rubber.
In addition to reinforcements, fibrils may be combined with a matrix to create composites having enhanced thermal, and/or electrical conductivity, and/or optical properties. They can be used to increase the surface area of a double layer capacitor plate or electrode. They can also be formed into a mat (e.g., a paper or bonded non woven fabric ) and used as a filter, insulation (e.g., for absorbing heat or sound), reinforcement, or adhered to the surface of carbon black to form xe2x80x9cfuzzyxe2x80x9d carbon black. Moreover, the fibrils can be used as an adsorbent, e.g., for chromatographic separations.
Fibrils are advantageously prepared by contacting a carbon-containing gas with a metal catalyst in a reactor at temperature and other conditions sufficient to produce them with the above-described morphology. Reaction temperatures are 400-850xc2x0 C., more preferably 600-750xc2x0 C. Fibrils are preferably prepared continuously by bringing the reactor to the reaction temperature, adding metal catalyst particles, and then continuously contacting the catalyst with the carbon-containing gas.
Examples of suitable feed gases include aliphatic hydrocarbons, e.g., ethylene, propylene, propane, and methane; carbon monoxide; aromatic hydrocarbons, e.g., benzene, naphthalene, and toluene; and oxygenated hydrocarbons.
Preferred catalysts contain iron and, preferably, at least one element chosen from Group V (e.g., molybdenum,tungsten, or chromium), VII (e.g., manganese), or the lanthanides (e.g., cerium). The catalyst, which is preferably in the form of metal particles, may be deposited on a support, e.g., alumina and magnesia.
The carbon fibrils have a length-to-diameter ratio of at least 5, and more preferably at least 100. Even more preferred are fibrils whose length-to-diameter ratio is at least 1000. The wall thickness of the fibrils is about 0.1 to 0.4 times the fibril external diameter.
The external diameter of the fibrils preferably is between 3.5 and 75 nanometers, i.e. determined by the particular application envisioned) have diameters within the range of 3.5-75 nanometers. Preferably a large proportion have diameters falling within this range. In applications where high strength fibrils are needed (e.g., where the fibrils are used as reinforcements), the external fibril diameter is preferably constant over its length.
Fibrils may be prepared as aggregates having various macroscopic morphologies (as determined by scanning electron microscopy) in which they are randomly entangled with each other to form entangled balls of fibrils resembling bird nest (xe2x80x9cBNxe2x80x9d); or as aggregates consisting of bundles of straight to slightly bent or kinked carbon fibrils having substantially the same relative orientation, and having the appearance of combed yarn (xe2x80x9cCYxe2x80x9d) e.g., the longitudinal axis of each fibril (despite individual bends or kinks) extends in the same direction as that of the surrounding fibrils in the bundles; or, as, aggregates consisting of straight to slightly bent or kinked fibrils which are loosely entangled with each other to form an xe2x80x9copen netxe2x80x9d (xe2x80x9cONxe2x80x9d) structure. In open net structures the degree of fibril entanglement is greater than observed in the combed yarn aggregates (in which the individual fibrils have substantially the same relative orientation) but less than that of bird nest. CY and ON aggregates are more readily dispersed than BN making them useful in composite fabrication where uniform properties throughout the structure are desired. The substantial linearity of the individual fibril strands also makes the aggregates useful in EMI shielding and electrical applications.
The macroscopic morphology of the aggregate is controlled by the choice of catalyst support. Spherical supports grow fibrils in all directions leading to the formation of bird nest aggregates. Combed yarn and open nest aggregates are prepared using supports having one or more readily cleavable planar surfaces, e.g., an iron or iron-containing metal catalyst particle deposited on a support material having one or more readily cleavable surfaces and a surface area of at least 1 square meters per gram.
Preferred support materials include activated alumina or magnesia in the form of aggregates of tabular, prismatic, or platelet crystals. Such material is commercially available, e.g., from ALCOA (in the case of activated alumina) and Martin Marietta (in the case of magnesia). The activated alumina supports yield primarily combed yarn aggregates, while the magnesia supports yield primarily open net aggregates. Spherical gamma alumina particles, which yield bird nest aggregates, are available from Degussa.
It is believed that deposition of a catalyst on a support consisting of readily cleavable planar surfaces allows the fibrils to assist each other as they grow, creating a xe2x80x9cneighborxe2x80x9d effect. As the catalyst particles deposited on the flat surfaces initiate fibril growth, the individual fibrils are influenced by their xe2x80x9cneighborsxe2x80x9d. In the case of the activated alumina support, this leads to a combed yarn fibril aggregate in which the individual fibrils have the same relative orientation. The magnesia supports, although having readily cleavable planar surfaces, yield primarily lightly entangled, open net fibril aggregates because they break apart more readily into smaller particles than the activated alumina support during fibril growth, resulting in aggregates that are less ordered than the combed yarn aggregates but more ordered than the tightly entangled fibril balls. The oxide precursors used to generate the metal catalyst particles also affect the tendency of the support to break apart. The more readily the oxide and support can form a mixed oxide at the interface between them, the more likely the support is to break apart.
Further details regarding the formation of carbon fibril aggregates may be found in the disclosure of Snyder et al., U.S. patent application Ser. No. 149,573, filed Jan. 28, 1988, and PCT Application No. US 89/00322, filed Jan. 28, 1989 (xe2x80x9cCarbon Fibrilsxe2x80x9d) WO 89/07163, and Moy et al., U.S. patent application Ser. No. 413,837 filed Sep. 28, 1989 and PCT Application No. US90/05498, filed Sep. 27, 1990 (xe2x80x9cFibril Aggregates and Method of Making Samexe2x80x9d) WO 91/05089, all of which are assigned to the same assignee as the invention here and are hereby incorporated by reference.
Fibrils are increasingly important in a variety of industrial uses and will become more so as these unique properties become better understood and exploited. While known methods of manufacture permit production of small quantities of fibrils, it is important to improve these methods, and in particular the catalysts used in those methods, to increase the yield of fibrils, to improve their quality and to lower their cost of production. It is also desirable to produce carbon fibrils of improved purity.
It is thus a primary object of the invention to provide improved catalysts for the production of fibrils.
It is a further object of the invention to increase the yield and productivity of fibril-producing catalysts.
It is still a further object of the invention to provide improved methods of preparing fibril-producing catalysts.
It is yet another object of this invention to improve the quality and uniformity of fibrils and aggregates thereof.
It is a further and related object of the invention to provide catalysts which lend themselves to large-scale fibril-producing processes.
It is still a further and related object of the invention to improve the economics and reliability of fibril manufacture.
Methods have now been found which substantially improve the yield of catalysts for the manufacture of carbon fibrils and carbon fibril aggregates. Substantially improved yields can be obtained by contacting a fibril-forming catalyst with an effective yield-enhancing amount of a carboxylate, such as a lower carboxylic acid or a salt thereof. The method is preferably carried out by precipitating an effective amount of a fibril-producing metal ion from an aqueous solution onto slurried particles of a support material in the presence of a carboxylate, such as an anion of a water-soluble carboxylic acid, preferably having 1 to 4 carbon atoms.
Another method which has been found to substantially improve the manufacture of catalysts for the production of carbon fibrils and fibril aggregates includes the step of coprecipitating a compound of a metal having fibril-forming catalytic properties and an aluminum or magnesium compound under controlled pH conditions to form a catalyst comprising a compound of aluminum or magnesium and a compound of the metal(s). Desirably an aqueous solution of an iron and/or molybdenum salt and an aluminum salt is formed and the metals coprecipitated to form a mixed oxide catalyst.
It has also been found that fibril-forming catalysts may be advantageously supported on carbon particles and desirably on carbon fibril aggregates composed of carbon fibrils of particular characteristics. In these aggregates, a preponderance of the fibrils have a length to diameter ratio of at least 5, an external diameter from 3.5 to 75 nanometers and a wall thickness of 0.1 to 0.4 times the external diameter. The fibrils have graphitic layers substantially parallel to the fibril axis and are substantially free of pyrolytically deposited carbon. The active fibril-forming metal is desirably iron or iron and molybdenum and preferably these active metal catalysts are deposited on the fibril aggregates as mixed oxides with alumina or magnesia as described above.
The improved methods of making fibril-forming catalysts and the improved catalysts themselves substantially increase the yield of fibrils per unit of fibril-forming catalytic metal. Carboxylate treatment during the making of fibril-forming catalysts yields catalysts with higher productivity. Coprecipitation of catalyst metals(s) and aluminum or magnesium compounds provides catalyst with higher loadings of active metal(s) and therefore higher productivities. Further, use of fibril aggregates as catalyst supports lend themselves to large scale fibril producing processes. The improved catalysts of the invention can be used to produce not only fibrils such as are described in Tennent, U.S. Pat. No. 4,663,230xe2x80x94although the fibrils of the invention have higher purity as madexe2x80x94but also fibrils having different macromorphologies, such as the so-called fishbone (xe2x80x9cFBxe2x80x9d) morphology as described in published European Patent Application No. 198,558 to J. W. Geus (published Oct. 22, 1986).
The term xe2x80x9cfibril-forming catalystxe2x80x9d is used to refer collectively to catalysts for forming discrete carbon fibrils, carbon fibril aggregates or both.
The term xe2x80x9ccarbon fibrilsxe2x80x9d when referring to products is used to refer collectively to both discrete carbon fibrils and carbon fibril aggregates, unless the context indicates a different meaning.
Carboxylate Treatment
Catalysts for producing carbon fibrils are made by incorporating an effective yield-enhancing amount of an anion capable of an anion exchange reaction with OH-groups, such as a carboxylate or a phenolate into a catalyst. Accordingly, catalysts for the production of carbon fibrils can be obtained by precipitating an effective amount of a compound of a fibril-producing metal, e.g. iron or iron and molybdenum, from an aqueous solution onto slurried particles of a support in the presence of an effective, yield-enhancing amount of a carboxylate. Preferably the carboxylate may generally be an anion of a water-soluble carboxylic acid, such as a substituted or unsubstituted, mono-, di-, tri- or polycarboxylic acid, preferably a water-soluble monocarboxylic acid having 1 to 4 carbon atoms, or an aqueous emulsion of a nonwater-soluble carboxylic acid with an emulsifier.
In alternative embodiments the carboxylate may be present in undissociated form provided that the carboxylate is capable of reacting with or otherwise affecting the surface properties of the metal oxide or metal hydroxide catalyst support. Hence, the carboxylate can be used in a nonaqueous solvent system that is also amenable to use as the solvent system in the formation of fibril-forming catalysts, such as, for example, an alcohol solvent system.
The carboxylate may be in the form of a hydrogen (i.e., carboxylic acid), sodium, potassium, ammonium or substituted ammonium, such as N-alkyl ammonium, carboxylate.
Desirably the anion is acetate or formate and is obtained from a water soluble salt of formic acid or acetic acid. Other carboxylates include propionates, butyrates, oxalates, citrates and tartrates.
A preferred form of the carboxylate is as the ammonium salt because ammonium ion is destroyed upon drying. Other carboxylate salts such as sodium, potassium and substituted quaternary ammonium, such as N-alkyl ammonium, may also be used.
A preferred method of carboxylate treatment includes the steps of (a) forming an aqueous solution of iron or iron and molybdenum salts, (b) forming a slurry of a catalyst support, e.g. alumina and or magnesia, (c) precipitating an iron compound or iron and molybdenum compounds onto the slurried particles of alumina and/or magnesia in the presence of an effective yield-enhancing amount of the carboxylate, preferably the anion of a lower carboxylic acid, at a pH at which precipitation is initiated and the precipitate maintained in insoluble form and generally in the range of 3 to 14, preferably 5.5-6.5, and (d) further processing it to produce the fibril-forming catalyst. Such further processing may comprise separating the so-impregnated support material from the slurry, drying it and finely dividing it or injecting the slurry directly into the reaction vessel for in situ conversion into catalyst.
In a preferred alternative embodiment, the method includes the steps of (a) forming an aqueous solution of iron or iron and molybdenum salt, (b) forming a solution of catalyst support precursor, such as an aluminum or magnesium salt, (c) mixing these two solutions and coprecipitating a mixed oxide catalyst in the presence of carboxylate and (d) separating the coprecipitated mixed oxide catalyst and further processing it. In a variant of this method, the alumina or magnesia support itself is formed in the presence of carboxylate and the catalyst is then precipitated onto the thus-created support from an aqueous solution of the catalyst precursor(s).
Desirably the solution from which the compounds of fibril-forming metal are precipitated onto the slurried particles of support particles contains from 0.04 to 4 grams of the anion of the carboxylic acid per gram of supported fibril-forming catalyst and preferably from 0.8 to 2 grams. Where iron or iron and molybdenum is the active fibril-forming metal, the weight ratio of anion of carboxylic acid to iron or iron and molybdenum in the solution from which the iron or iron and molybdenum is precipitated is broadly in the range of 0.07 to 14 and preferably is in the range of 1.4 to 5.2.
In preferred embodiments where the carboxylic acid is acetic acid, the weight ratio of acetate to iron in the solution is in the range of 0.1 to 5. Acetate is the preferred anion when precipitation is carried out at a controlled pH among other reasons because solutions thereof act as buffers.
The precipitated metal on catalyst support is filtered and may be washed and reslurried and then vacuum or pressure filtered. The washed and filtered slurry is then dried and may be ground to xe2x88x92100 mesh and thereafter tested for productivity. The catalyst is activated by reduction of the compounds of fibril-forming catalyst metal to the corresponding catalyst metal. The catalyst can also be prereduced prior to use.
It has been found that it is advantageous to introduce the carboxylate salt, e.g. of acetic or formic acid, into the slurry of the support material prior to its combination with the solution containing the active fibril-forming metal compound.
While the precipitation procedure works best if the carboxylate is present in the slurry of catalyst support material, e.g. alumina, the support material can also be pretreated with a solution of carboxylate and then dried before the iron or iron and molybdenum salt is precipitated thereupon. A post-precipitation contact with carboxylate is less effective, but still provides improved results over catalysts without carboxylate treatment.
On magnesia-supported catalysts, still other procedures for treating with carboxylate work well. For example, washing a magnesia-supported catalyst with 1N ammonium acetate after precipitation of the iron or iron and molybdenum compound(s), i.e. a post-precipitation method, has a strong positive effect on catalyst productivity.
While not wishing to be bound by any theory, it is believed that the carboxylate acts by exchanging with surface hydroxyl ions to alter the surface characteristics of the support material. This, in turn, can strongly affect the attachment of the small iron oxide or iron/molybdenum oxide particles to the surface of the support. With magnesia supports, the ion exchange can be accomplished efficiently by washing after the iron oxide or iron and molybdenum oxides have been deposited. It is believed that surface modification of the support by carboxylate can be beneficial to the catalyst in two ways. First, the carboxylate, by affecting the attachment of the metal oxide particles to the surface, makes the subsequent reduction and activation of those particles take place more efficiently. Secondly, the surface modification changes the friability characteristics of the support surface which can have a beneficial effect.
The improvement in yield achieved with carboxylate-treated catalysts ranges from about 10 to 20% when preparing bird nest (BN) type fibril aggregates to about 100% when producing combed yarn (CY) or open net (ON) type fibril aggregates.
Co-Precipitation
Improved fibril-making catalysts can be obtained by coprecipitating the mixed oxide(s) of fibril-forming active metal and a precursor of a second oxide, e.g. aluminum oxide or magnesium oxide. In a preferred embodiment an aqueous solution is formed comprising (i) a salt(s) of one or more metal(s) having fibril-forming catalytic properties and (ii) an aluminum and/or magnesium salt(s). The fibril-forming metal(s) and the aluminum and/or magnesium are coprecipitated from the aqueous solution as the mixed oxides of the metal(s) and aluminum and/or magnesium and thereafter the precipitate is filtered, washed, dried and ground as is known in the art.
Desirably the fibril-forming metal catalysts are iron or iron and molybdenum and these are precipitated by the addition of a base such as ammonium carbonate or sodium carbonate at a pH sufficient to initiate precipitation and to maintain the precipitate in insoluble form. The pH is generally in the range of from 3 to 14 and preferably is in the range of 5.5 to 6.5. Precipitation preferably is carried out in the presence of a yield-enhancing amount of an anion of a water-soluble carboxylic acid.
The solution from which the iron compound or iron and molybdenum compounds are precipitated desirably contains from 0.01 to 1 gram of iron, preferably from 0.3 to 0.5 gram; from 0.005 to 0.25 gram of molybdenum, preferably from 0.06 to 0.1 gram; and from 0.1 to 1 gram of aluminum and/or magnesium, preferably from 0.2 to 0.5 gram, per gram of finished fibril-forming catalyst. The pH is sufficient to maintain the iron compound or iron and molybdenum compounds in solution is broadly from 0 to 3 and preferably from 0 to 1.
Carbon Supports
Carbon particles having a high degree of structure and an open pore structure with high surface area can be used as supports for the mixed oxide catalysts of the invention. Carbon particles having a high porosity, i.e., a low bulk density, and a high surface area are said to have a high degree of structure.
Carbon particles that are essentially pure carbon are preferred since they add the least amount of contamination to the final fibril product. Examples of these materials are (1) carbon blacks available from Cabot Corp., such as the REGAL, VULCAN, MONARCH and ELFTEX series of carbon blacks described in Cabot Technical Report S-134 and (2) vapor-grown carbon fibers, such as reported by M. Endo, et al. in xe2x80x9cThe Extended Abstracts of the 18th Biennial Conference on Carbonxe2x80x9d, Worcester, Mass. (American Carbon Society, University Park) at p. 151; by G. G. Tibbits, ibid. at p. 157, by Asahi Chemical Ind KK in Japanese Patent No. 62-263,377 (May 6, 1986), by Showa Denka KK in Japanese Patent No. 62-078,217 (Sep. 26, 1985); or by Nikkiso KK in Japanese Patent No. 61-070014 (Apr. 10, 1986). These carbon fiber materials may also include pyrolytically deposited carbon and, optionally, may be graphitized in a further processing step at  greater than 2500xc2x0 C. Commercially available reinforcement fibers, e.g., those made from poly(acrylonitrile) fibers, are also candidates. Any such material should be selected so as not to adversely affect the properties of the fibrils made using the material as a catalyst support.
More preferred are carbon particles as described above with high surface area, i.e,  greater than 250 m2/g, since these are more amenable to use as catalyst supports, i.e, they possess the surface area, porosity, pore structures and handleability necessary to prepare catalysts for commercial operation. Examples of these materials are the activated carbons and activated charcoals, such as the WV-B, WV-W, WV-L, or WV-G series materials available from Westvaco or the MONARCH BLACK PEARLS or VULCAN XC72 materials available from Cabot Corp.
Most preferred are carbon fibril aggregates since they combine the high purity and high surface area of the materials mentioned above and, additionally, present a uniquely high macro-porosity (up to 8 cc/g) with an open pore structure, i.e, a pore structure with essentially no micropores (diameters  greater than 2 nm). Micropores often render the internal surface area of a carbon material inaccessible to reactants because of diffusion limitations and make the particle often subject to plugging. Fibril aggregates with their unique porosity and relative absence of micropores do not suffer this shortcoming. Excellent results are obtained with fibril aggregates as supports for the mixed oxides of fibril-forming metal(s) and aluminum or magnesium.
The alumina or magnesia can be formed in situ using the fibril aggregate as the sole support, or, aluminum or magnesium oxides can be added to the aggregates as a well-dispersed slurry before deposition of the active metal catalysts. The coprecipitation procedure described above for iron oxide or iron and molybdenum oxide and aluminum oxide may be carried out in the presence of the carbon particles or fibril aggregates. Where an aluminum and/or magnesium compound and the fibril-forming metal are coprecipitated onto an aggregate or carbon particle support, the aqueous solution of fibril-forming metal and an aluminum and/or magnesium compound contains from 0.01 to 1, preferably 0.2 to 0.5, gram of iron, from 0.005 to 0.25, preferably 0.05 to 0.1, gram of molybdenum and from 0.01 to 1, preferably from 0.2 to 0.5, gram of aluminum and/or magnesium per gram of supported fibril-forming catalyst and the slurry of fibril aggregates or carbon particles has from 0.01 to 0.9, preferably from 0.4 to 0.7, gram of aggregate or carbon particles per gram of supported fibril-forming catalyst.
The aggregates are, for example, the BN, CY or ON aggregates described above although other aggregates are also useful, including aggregates of fishbone (xe2x80x9cFBxe2x80x9d) morphology characterized by a crystalline graphitic structure and a morphology defined by a fishbone-like arrangement of the graphite layers along the axis of the filaments. Fibril aggregates are desirable owing to their high surface area e.g., from about 250 to about 1000 m2/gram and preferably greater than about 250 m2/gram, and their unique macroscopic porosity of up to about 8 cc/gram, or typically in excess of 1 cc/gram and preferably in excess of 5 cc/gram.
It has been found that an effective catalyst cannot be prepared by separately coprecipitating the mixed metal oxides and then physically mixing them with fibril supports. The latter procedure results in a mixture of materials having greatly different bulk densities such that when the mixtures are filtered, the solids partially separate and a non-homogenous mixture is obtained.
Coprecipitation in the presence of fibril aggregates gives a uniform homogenous distribution of active catalyst strongly attached either chemically or physically to the aggregate. It is thought that the small particles of the metal oxides and the alumina and/or magnesia deposit in the crevices and pores of the intertwining carbon fibrils in the fibril aggregate and become strongly fixed thereto. The alumina or magnesia provides for the stable deposition of the iron oxide or iron and molybdenum oxides. Were it not present, the iron and molybdenum particles formed by reduction of the oxides would be too mobile on the graphite surface of the fibril aggregate at the temperatures required for fibril growth and would fuse into large particles and deactivate before starting to grow carbon fibrils.
A significant advantage of coprecipitation in the presence of fibril aggregates, or, to a lesser extent, incorporating a fibril-forming metal onto magnesia particles in fibril aggregates, is that the amount of alumina or magnesia in the catalyst is significantly reduced. This decrease results in a higher yield of carbon fibrils based on alumina or magnesia content and decreases the amount of alumina or magnesia impurity included in the product. While the amount of alumina or magnesia decreases, the yield per unit of fibril-forming metal remains the same. The amount of washing that is needed to remove alumina or magnesia from the carbon fibril product is also reduced.
Where magnesia is used, catalysts can be made by finely dispersing magnesium oxide with fibril aggregates or carbon particles in water to form a slurry and adding a solution of iron or iron and molybdenum salts, such as ferric nitrate and ammonium molybdate. Where the fibril-forming metal is incorporated onto magnesia particles in fibril aggregates or carbon particles, the aqueous solution of fibril-forming metal contains from 0.01 to 1, preferably 0.2 to 0.5, gram of iron and from 0.005 to 0.25, preferably 0.05 to 0.1, gram of molybdenum per gram of fibril-forming catalyst and the slurry of magnesia particles and fibril aggregates or carbon particles contains from 0.01 to 1, preferably from 0.2 to 0.5, gram of magnesia and from 0.01 to 0.9, preferably 0.4 to 0.7 gram of fibril aggregates or carbon particles per gram of supported fibril-forming catalyst. The homogeneity of the resulting catalyst is evident upon filtration. A well-dispersed, homogeneous solid catalyst has a regular coloration, while poorly dispersed catalysts, which undergo partial separation of solid components on filtering, have black and gray striations in the filter cake.
Fine aqueous dispersion of magnesia is possible because of a physical attraction between the magnesium oxide/hydroxide and the fibril surface. However, dispersability also depends on the starting material from which the magnesia dispersion is prepared. If the density of the magnesium oxide is too high or the dispersibility is too low, non-homogeneous catalysts can result. Therefore, procedures to fully disperse or homogenize the fibril aggregates and magnesium oxide must be undertaken before neutralization of the iron or iron and molybdenum oxides is carried out. Such methods of dispersion or homogenization are known to the art.
Where alumina is used, catalysts can be made by dispersing fibril aggregates in water and coprecipitating the oxides of iron, aluminum and molybdenum from a solution containing ferric nitrate, aluminum nitrate and ammonium molybdate while maintaining the pH at 6.0xc2x10.5 by concurrent addition of a solution of ammonium carbonate. The homogeneity of the catalyst is excellent as judged by coloration.
The supported fibril-forming catalyst of the invention comprises from about 1 to about 70 weight percent, preferably from 5 to 50 weight percent and more preferably from 12 to 40 weight percent, of iron or iron and molybdenum; from 1 to 95 weight percent, preferably from 10 to 85 weight percent and more preferably from 20 to 80 weight percent, of alumina and/or magnesia; and from 1 to 90 weight percent, preferably from 20 to 70 weight percent and more preferably from 30 to 50 weight percent, of carbon fibril aggregates or carbon particles based on the total supported fibril-forming catalyst weight, with the proviso that the total weight percent of iron or iron and molybdenum alumina or magnesia and carbon fibril aggregates or carbon particles does not exceed 100 weight percent.
The carbon fibrils of the invention that are made using the more preferred supported fibril-forming catalysts have extremely high purity, as madexe2x80x94to the extent that they are essentially pure carbon. In these carbon fibrils the impurity level from the fibril-forming catalytic metal is not more than about 1.1 weight percent, that from the alumina or magnesia support is not more than about 5 weight percent and the total impurity level is not more than about 6 weight percent. This provides a higher purity product. The previous levels of impurities in making fibrils were about 1.5 weight percent from the fibril-forming catalytic metal, about 8.5 weight percent from the alumina or magnesia support and about 10 weight percent for total impurities. The purity of the fibrils as made in the invention reduces the amount of washing needed to prepare the fibrils for sale and their cost. The impurities from the fibril-forming catalytic metal occur mainly as particles of metal that have been encapsulated within the carbon fibril so that they are not exposed.